It is known in the prior art for tactile, but not optical, sensors for coordinate measurement machines to be equipped with collision protection. By way of example, EP 0 426 492 A2 discloses a tactile sensor which is intended for continuous measurement of workpieces and has a sensor element which is held in an equilibrium position by restoring forces. The restoring forces are produced by springs. The sensor element is held, preferably magnetically, on a sensor holder via precision bearings, with the restoring forces acting on the sensor holder.
In contrast to this tactile sensor, an optical sensor is moved past the surface of a workpiece to be measured, without touching it. There is therefore no need to hold the sensor in an equilibrium position via restoring forces. In fact, an optical sensor is attached to the sensor holder without any play.
U.S. Pat. No. 5,404,649 discloses a further tactile sensor for a coordinate measurement machine. The sensor is held on a sensor receptacle by a magnet. An isostatic three-point rest is provided between the sensor and the sensor receptacle. The coupling between the sensor and the sensor receptacle is designed such that the sensor is moved away from an initial position in the event of a collision with a workpiece, and snapped back to its initial position because of the magnetic attraction forces when the sensor is free again, irrespective of whether this is because it has moved back or because the workpiece has been removed.
In contrast to the situation with sensors which make contact, optical sensors have optical waveguides which are sensitive to mechanical loading and must be routed in the sensor. The coupling from U.S. Pat. No. 5,404,649 has the disadvantage that the magnet is arranged centrally, making it more difficult to route the optical waveguide in a manner which protects it.
In comparison to the known sensors which make contact, new problems arise for an optical sensor because of the optical waveguide and the fact that the sensor element is not in contact with workpieces to be measured during normal operation.